In a large mixing bowl combine the garlic, cumin, oregano, red pepper flakes, 1 tsp salt and ½ tsp black pepper.
Pat the beef dry with paper towels and place in the bowl with the spices. Turn the beef to coat all sides with the spices.
Heat the oil in a dutch oven or large saute pan over high heat until shimmering. Sear the beef in batches on all sides until well browned.
Place the onions in the bottom of the slow cooker, add the seared beef and then top with the green chiles.
Cover the slow cooker and cook on low for 8 hours.
Fill warmed tortillas with the beef and toppings of your choice.
